When it is mounted in an aircraft, such an anemometer measures the speed of movement of the still air relative to the aircraft, at a distance, i.e. the speed of the aircraft itself with respect to the still air. Its operating principle is based on the phenomenon known by the "Doppler effect".
Anemometers of the above defined type are already known, which do not comprise means for moving the incident beam in space or comprise a continuous conical scanning system.
For a fixed beam anemometer, the volume in which the relative speed of the particles is measured is an elongate volume extending substantially along the axis of the incident beam and it is in fact the projection of the relative speed along this axis which is measured. The information delivered by such an anemometer is therefore partial, since it relates to only one component of the relative speed to be measured.
To partly overcome this disadvantage, means may be provided for varying the direction of the incident beam. For example, it is known to use a prism intercepting the incident beam and mounted for pivoting about an axis perpendicular to its input face, which axis merges with the axis of the incident beam. At rest, the incident beam is therefore deviated by the prism. When the prism is rotated, the incident beam scans a cone, which makes it possible to make measurements in different directions. However, the half angle at the apex of the scanned cone remains small. The result is that the accuracy obtained on the transverse components of the relative speed is low. In addition, the measurement of the speed in a given direction is not instantaneous. It is therefore necessary either to drive the prism intermittently, leaving it stopped during each measurement, or to drive it continuously, but at a fairly slow speed. In both cases, that involves a compromise between the rapidity and the accuracy of the measurements. Moreover, such an anemometer is not very easy to install in an aircraft, to the extent that, if it cannot be installed at the end of the nose of the aircraft, a window transparent to the laser beam would have to be provided in the form of a vertical ring going right round the fuselage, so as to let the laser beam pass through which scans the cone with a substantially horizontal axis. This does not seem reasonably possible.